1. Field of the Invention
This invention relates to a filter element sealing device for a filter pan and more particularly to a sealing device for a filter pan used in the manufacture of various products such as phosphoric acid from phosphoric ore.
2. Description of the Prior Art
Because of the ever increasing demand for phosphates as agricultural soil fertilizers, the manufacture of phosphates from phosphoric ore has become a major industry. Various processes have been employed in obtaining phosphates from the phosphoric ores but these processes generally fall into two categories: firstly, the elemental phosphorus process and secondly, the wet acid process. The wet acid process involves crushing and grading raw phosphate rock known as Fluorapatite and treating this crushed rock with an acid such as sulphuric acid to obtain a single, super-phosphate which is a mixture of calcium sulphate and monocalcium phosphate.
In the wet acid process, the crushed Fluorapatite is treated with sulphuric acid in a digester. Slurry from the digester is then fed to a filtration unit where phosphoric acid is separated from the calcium sulphate precipitate. This separation is carried out by the application of partial vacuum applied to the underside of the filter element.
A typical filtration unit utilized in the phosphate industry for the separation of calcium sulphate is of generally circular configuration. The filtration unit includes twenty-four filter pan cells of substantially trapezoidal shape arranged as partial sectors of the circular filtration unit. Each filter pan cell is provided with means to enable the pan to rotate about a radial axis of the circular unit.
The filter pans are driven about a vertical axis of the circular unit so that each filter pan will progressively pass through various stations. At the first station the pan is filled with feed slurry from the digester. When the pan has been filled, partial vacuum is applied to the underside of the filter element to draw off undiluted mother liquor, which is phosphoric acid. At the next station, the filter pan, which now contains a filter cake of calcium sulphate, is sprayed with a weak wash liquor returned from the operations performed on the filter cake at the next station. Vacuum is again applied to the underside of the filter element to draw off the strong wash liquor having a concentraton of phosphoric acid less than the undiluted mother liquor extracted at the first station. During the next station wash water is applied to the calcium sulphate filter cake while vacuum is applied to extract a weak wash liquor which is returned to the second station for spraying onto the filter cake. The next station includes rotation of the filter pan cell to invert the pan to empty the filter cake therefrom. At this station, instead of a vacuum, air is blown through the underside of the filter element to dislodge the filter cake. At the following station, water is sprayed upwardly into the inverted filter pan to clean the pan of calcium sulphate precipitate. The pan is then dried and ready to receive the next supply of slurry from the digester.
The above filtration unit thus operates on a continuous basis with the filter pans rotating through all of the above stations every three to five minutes. During the above filtraton process, due to the large quantities of slurry being handled, the filter element is often damaged and this necessitates the removal of the filter element from the filter pan and the subsequent replacement of the filter element. Because of the corrosive nature of the calcium sulphate precipitate and because of the manner in which the precipitate rapidly hardens, serious problems have been experienced when trying to change filter elements as the precipitate manages to clog the joint between the filter element and the filter pan.
U.S. Pat. Re No. 24,150 to Delruelle relates to a continuous rotary filter of the tilting cell type in which the filter cloth is secured to the base of the pan cell by a rubber sealing strip of circular cross-section inserted within a correspondingly shaped undercut groove in the base of the pan cell.
U.S. Pat. No. 3,426,909 to Garner describes a filter element anchoring means including two upright members each having a bulbous portion at the upper end thereof. The filter cloth is inserted within an elongated pocket formed between the bulbous portions and is locked within this pocket by a resilient caulking strip of rope or rubber.
U.S. Pat. No. 3,080,063 to Kryrski et al discloses a channel-shaped groove for retaining a filter element therein. The filter element is held within the groove by caulking material.
None of the above prior art teaches a positive means other than the caulking material itself for retaining the filter cloth within the groove or pocket.
Serious problems have been experienced with the prior art sealing devices in that the calcium sulphate precipitate rapidly hardens to clog the caulking strip within the groove thus making the removal of the filter element extremely difficult.
Experience has shown that on the average one and one-quarter hours is required by a team of fitters to replace such a filter cloth and that these fitters have had to resort to the use of power chisels to dislodge hardened precipitate from the sealing ring. This downtime is not only costly in terms of man hours lost, but more importantly, the production of phosphates is lost while the whole unit is shut down for repairs.
An attempt has been made in the prior art to alleviate this problem by providing an annular groove in the wall of the filter pan. In this arrangement, a sealing ring urges the filter element within the annular groove and locks the filter element therein by means of a plate bolted to the sidewall of the pan. The above arrangement has proved unsatisfactory as the precipitate manages to enter the cracks between the sealing ring and the filter element where it lodges and hardens.
The present invention seeks to provide a simple and inexpensive means of overcoming the foregoing problems. The primary objective and advantage of the present invention is the provision of a sealing device which can easily be removed from the sidewall of a filter pan to facilitate replacement of a filter element.
A further objective and advantage of the present invention is the provision of a sealing device that effectively prevents the ingress of material past the seal.
Another object of the present invention is the provision of a sealing device which may be easily installed in a fraction of the time required by the prior art process.
Another object of the present invention is the provision of a sealing device which eliminates the necessity of grooves capable of trapping calcium sulphate precipitate.
Further objects and advantages of the present invention will become apparent from the study of the following portions of the specification, the claims and the attached drawings.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure particularly with regard to the use of the invention disclosed herein. This should not be construed as limited to a filter element sealing device for a filter pan but should include other applications in which any filter element is to be sealed at its edge.